Orpheus

How Much Concrete Do I Need?

Volume, bags and the waste allowance you will use.

Inputs
m
m
mm

Typical: 100 mm for a path or shed base, 150 mm for a driveway.

10%

Uneven subgrade, spillage and over-dig. Ten percent is the usual figure.

Concrete needed
1.32 m³
Without waste allowance
1.2 m³
Bags required
121 × 25 kg
Approximate weight
3.17 tonnes
In cubic yards
1.73 yd³
Area covered
12 m²
volume = length × width × depth, plus a waste allowance

121 bags is manageable by hand or with a small mixer. Order the waste allowance: running short mid-pour leaves a cold joint you cannot undo.

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Multiply length by width by depth in metres, then add about ten percent for waste. A 4 by 3 metre slab at 100 mm is 1.2 cubic metres, or 1.32 with the allowance.

How to use Concrete Calculator

  1. Measure the area. Length and width in metres, depth in millimetres.
  2. Add a waste allowance. Ten percent covers uneven subgrade and spillage.
  3. Read volume and bags. Bag counts assume the volume a bag of dry mix actually yields.

About calculating how much concrete is needed

Concrete quantities are straightforward volume arithmetic with one place where estimates reliably go wrong: the depth is never as uniform as the drawing. An excavation dug by hand or by machine varies by ten or twenty millimetres across its area, and every millimetre of extra depth across a four by three metre slab is another twelve litres of concrete. That is why a waste allowance is standard practice rather than padding, and why ten percent is the conventional figure. The bag conversion surprises people more than the volume does. A twenty-five kilogram bag of dry mix yields only about eleven litres of concrete once water is added, so a single cubic metre takes roughly ninety bags. Working that out is usually the moment a project moves from bags to ready-mix delivery, which past about a cubic metre and a half is both cheaper per unit and dramatically less work. The reason to order generously is that running short is not merely inconvenient. Concrete begins setting as soon as it is mixed, and fresh material poured against a batch that has already started to cure forms a cold joint — a permanent weak seam through the slab. A trip to buy more bags mid-pour is exactly how that happens. Depth is worth getting right for the same reason: a hundred millimetres suits paths and shed bases, and anything a vehicle will sit on wants a hundred and fifty over a properly compacted sub-base, which does as much structural work as the concrete above it.

Frequently asked questions

How deep should a slab be?
Around 100 mm for a garden path, shed base or patio, and 150 mm where vehicles will drive on it. A driveway taking heavy vehicles wants 150 to 200 mm over a properly compacted sub-base, and the sub-base matters as much as the concrete.
Why does a 25 kg bag make so little concrete?
Because you add water to it. Twenty-five kilograms of dry mix yields roughly 0.011 cubic metres once mixed, so a cubic metre takes about ninety bags. This is the calculation that usually convinces people to order ready-mix.
When should I order ready-mix instead?
Generally past about one and a half cubic metres. Beyond that the bag count runs into the hundreds, hand mixing takes a full day, and the risk of a cold joint from a slow pour becomes real. Ready-mix is also usually cheaper per cubic metre at that volume.
What is a cold joint and why does it matter?
A weak seam formed where fresh concrete is poured against a batch that has already begun to set. It happens when a pour stops for too long — running out of material is the usual cause — and it permanently reduces the strength of the slab.
Should I round up?
Always. Excess concrete is a minor waste; running out during a pour is a structural defect. The ten percent allowance covers ordinary variation in dig depth, which is almost never as even as the measurement suggests.

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